1. Field of the Invention
Aspects of the present invention relate to a method for preparing a positive active material for a lithium ion secondary battery, and more particularly, to a method for preparing a positive active material for a lithium ion secondary battery, which can improve life and safety characteristics of the battery by minimizing byproducts such as Li2CO3 and LiOH, a positive active material prepared thereby, and a lithium ion secondary battery including the same.
2. Description of the Related Art
Due to recent trends toward more compact and lighter portable electronic equipment, such as cellular phones, camcorders and notebook computers, there has been a growing demand for improved lithium ion secondary batteries of high performance, durability and reliability that can be used as power sources for the electronic equipment. In addition, as realization of electromotive vehicles is keenly interested, lithium ion secondary batteries are drawing particular attention as power sources for the electromotive vehicles.
A lithium ion secondary battery generally includes a positive electrode and a negative electrode that are capable of intercalating and deintercalating lithium ion, a separator that prevents the positive electrode and the negative electrode from physically contacting each other, and an organic electrolyte or polymer electrolyte that transmits lithium ion between the positive and negative electrodes. In the lithium ion secondary battery, when lithium ion is intercalated/deintercalated in the positive electrode and the negative electrode, electrical energy is generated through an electrochemical oxidation/reduction reaction.
In the general method for manufacturing a positive electrode, a lithium containing compound and metal oxide are mixed and heat treated at high temperature to prepare a positive active material. Next, the positive active material is mixed with a binder such as a PVDF resin in a solvent to prepare a slurry. The slurry is coated on both surfaces of a positive current collector made of, for example, an aluminum foil, and dried to prepare a positive electrode.
In order to provide for shortage in the amount of lithium contained in the positive active material due to volatilization of lithium ions during heat treating at high temperature in the course of manufacturing the positive electrode, a molar ratio of the lithium containing compound to the metal oxide is controlled such that an excess of the lithium containing compound is used relative to the metal oxide. For the reason stated above, some of the excessively used lithium compound is mostly diffused into the metal oxide, some of the lithium compound is volatilized, and the remainder on a surface of the metal oxide without being diffused into a metal crystal lattice is used to generate byproducts such as LiOH, LiHCO3, or Li2CO3 with H2O and O2 generated during a calcinating process.
In a case where excessive lithium ions exist on the surface of the lithium metal oxide as the positive active material without being intercalated/deintercalated into/from the metal crystal lattice even after repeated charge/discharge cycles, the excessive lithium ions may be dissolved or precipitated, causing dendrite growth. This may cause a short circuit and may further cause serious problems with the life and safety of the battery.